Look at this thread, I made a flow indicator very similar like your idea, just didn't have time to evolve it to flow sensor. The magnet and hall effect idea is good, but make sure that the magnets are strong enuff to trigger the hall eff. device outside of the box, and to balance the impeller (with magnets) properly.
And ofcourse, post the pics!!!
http://forums.procooling.com/vbb/sho...5&pagenumber=1